1. Field of the Invention
The present invention relates to metal vapor discharge lamps such as high-pressure sodium lamps in which sodium, mercury and a rare gas are enclosed in the arc tube.
2. Description of the Prior Art
Hereafter, a high-pressure sodium lamp as an example will be described.
As shown in FIG. 1, a high-pressure sodium lamp generally consists of an electric introducer (3) made of heat-resistant metal and an electrode (6) fixed to the electric introducer (3), with said electric introducer (3) and electrode (6) fitted in cap (2) comprising alumina ceramic, etc. using a glass frit (5), said cap (2) fitted into each end of an arc tube (1) made of alumina ceramic, etc. by means of glass frit (4), and sodium, mercury, and xenon (Xe) or other rare gas at several tens of Torr used as the starting gas are sealed inside thereof. In FIG. 1, the components marked by numbers with a dash (') denote similar components to those marked with the numbers without dash. Sodium lamps having a starting aid (12) equipped over the arc tube (1) to lower the starting voltage as shown in FIG. 2 are also known well. FIG. 2 is a mount diagram for a sodium lamp having a starting aid. In this lamp easy starting is ensured: metal frame wires (7) and (8) serving as input terminals are connected and fixed, via metal wires (13) and (14), respectively to electric introducers (3) and (3') which are made of heat-resistant metal and are located at both ends of the arc tube (1) consisting of alumina ceramic, etc.; a starting aid (12) consisting of heat-resistant metal wire is laid around the outer circumference of the arc tube (1), with both ends thereof being electrically insulated and held with glass beads (9) and (10); and only at the time of starting, either input terminal (input terminal (7) in FIG. 2) is connected electrically by means of a bimetal (11), so that the distance between the two electrodes at starting can be shortened to considerably reduce the starting voltage for easy lamp starting. Members (15) and (16) in FIG. 2 are input terminals connected to said metal frame wires (7) and (8), and member (17) a stem to support said input terminals (15) and (16).
In recent years, new sodium lamps have been proposed for improved color-rendering properties in which heat-resistant metal belts (18) as shown in FIG. 2 are wrapped around the ends of the arc tube (1) as heat insulators to heighten the temperature of the coolest sections at the ends of said arc tube (1). Metal belts (18) as mentioned herein keep the coolest sections of the arc tube (1) warm, raise the sodium's vapor pressure inside the arc tube (1), enhance the sodium's resonance absorption, and have the emission spectra spread over the whole visible range, thus improving the color-rendering properties. Such warmth-keeping effect shows itself as the lamp voltage among the lamp's electrical characteristics. FIG. 3 shows the relationship between the width a of said metal belt (18) and the polential gradient E (V/cm). The potential gradient is a value obtained by dividing the lamp voltage by the arc length (electrode-to-electrode distance), and is convenient as a factor used for different arc lengths, etc. It can be seen in the figure that a potential gradient of 12 V/cm in the case without a metal belt (18) (i.e., width a=0) can be raised to about 18 V/cm by increasing the width a to 5 mm. FIG. 4 shows the relationship between the potential gradient E (V/cm) of a high-pressure sodium lamp and its general color-rendering index Ra in the case where use is made of a xenon (Xe) pressure of 20 Torr and a sodium molar ratio of 0.74. Raising the potential gradient leads to an increased Ra, and an increased tube diameter also results in an increase in the Ra value. The latter method, however, is not commonly employed because the materials of arc tubes such as polycrystalline alumina are expensive. It is, therefore, general practice to use tubes of 10 mm.phi. or smaller diameter.
The color rendering properties of a high-pressure sodium lamp should be such that the Ra value is 40.about.70 or preferably 50.about.60. The reason is that Ra of 40 or below makes the lamp unsuitable as an indoor illuminating light source, while Ra of 70 or above causes a considerable reduction in the luminous efficacy. Therefore, an attempt to obtain an Ra of 40 using a tube of 8 mm.phi. diameter in FIG. 4 will result in a potential gradient E of 21 V/cm. In this case, the sealed section near the tube's coolest section will be at a temperature of about 770.degree. C., as seen from FIG. 4. To obtain an Ra of 60, which represents good color-rendering properties, a temperature of 800.degree. C. or above will have to be encountered at the sealed section. In FIG. 5, the thickness of the sodium diffusion layer inside a sealed glass, with said sealed glass and sodium having been put in a container and allowed to stand at several different treatment temperatures for a predetermined length of time, is plotted using said treatment temperature as the variant. (Refer to "Mitsubishi Denki Giho" p. 1177, vol. 47, No. 11, 1973)
It can be seen from FIG. 5 that over 750.degree. C. the sodium diffuses in a reacted form inside the sealed glass. Since the sealed glass treatment temperature in FIG. 5 can be regarded as equivalent to the temperature of the sealed section shown in FIG. 4, the latter temperature is required to be 750.degree. C. or below. That is, when the temperature of the sealed section in FIG. 4 rises above 750.degree. C., the sodium reacts with the sealed glass, which in turn renders the sealed glass brittle, thus shortening the service life of the lamp. Although the general color-rendering index Ra has a relation with the sodium molar ratio as well as with the potential gradient E and tube diameter, the relationship between the sealed section temperature and Ra shown in FIG. 4 is considered not to change considerably. That is, an attempt to obtain an Ra of 60 at 750.degree. C. or below has obliged the use of a large, expensive arc tube of the 12 mm diameter class, as indicated in FIG. 4.
There is another method available for improving the color-rendering properties of a high-pressure sodium lamp: the vapor pressure of the sodium during lighting is raised to have the sodium itself absorb the radiation of the Na-D lines (5896 and 5890 .ANG.) and re-radiate from different energy levels, so that the broadening of the sodium D lines can be promoted to five radiation spectra spread almost all over the whole visible range. However, since the emission spectra spread almost all over the visible range reduce the percentage of emission in the wavelength range near 555 mm with high spectral luminous efficacy, this method has the drawback that it gives lower spectral luminous efficacy than conventional high-pressure sodium lamps.
The luminous efficacy of a lamp .eta. (lm/W) is expressed by: EQU .eta.=.eta..sub.e .multidot.K (1)
where K (lm/W) demotes the visual luminous efficacy and .eta..sub.e the radiation efficiency of the visible region. K and .eta..sub.e are given by the following formula: ##EQU1## where V (.lambda.) denotes the spectral visual co-efficient, and P.lambda. the spectro-radiation energy. The value of K is about 400 lm/W in ordinary high-pressure sodium lamps, but it falls to about 330 lm/W by efforts to improve the color-rendering properties. .eta..sub.e is about 0.3, with almost no difference between the ordinary and high color-rendering types. As a whole, therefore, ordinary high-pressure sodium lamps have luminous efficacy of .eta.=400.times.0.3=120 lm/W, but high color-rendering type lamps reduced efficacy of about .eta.=330.times.0.3=99 lm/W. That is, although either K or .eta..sub.e or both of them can be increased to raise the efficacy .eta., some limitation is placed on the value of K to obtain the desired color-rendering properties, because the visual luminous efficacy K has a close connection with the sodium vapor pressure. Therefore, .eta..sub.e should be changed primarily. The visible radiation efficiency .eta..sub.e is related with the visible radiation energy transmittance of the arc tube, arc thermal conduction loss in the arc tube, and other factors. Furthermore, high-pressure sodium lamps have had the drawback that there is a considerable scattering in the drop of the starting voltage by starting aid (12), thus resulting in unfixed starting voltage.
The present invention was devised in view of the above-mentioned drawbacks.